The present invention relates to the field of arc detection systems. More specifically, in one embodiment the invention provides a means for detecting arc in an electrical system and for disconnecting the power source supplying the power which sustains the arc when the arc is detected.
A common problem in electrical systems, especially those operating at high voltages, is the interruption of an electrical conduction path and subsequent formation of arcs. This occurs when a wire frays or a circuit element or trace breaks. Since a typical break leaves only a small gap, any current on the path might continue to flow through the path by arcing across the gap. The temperature of the arc is very high, and consequently, arcing in electrical systems could lead to fires, especially where the arc occurs within a piece of electronic equipment, where components and circuits tend to be tightly spaced. Another place where electrical fires are a problem is in electrical wiring in homes and businesses, since the wiring is often in contact with flammable materials such as insulation, carpeting, and wood.
When a break in a conduction path occurs, no current flows. Once an arc is established, current flows steadily. But while the arc is being established, the current pulses as a series of quick arcs leap across the gap and ionize the air over the gap. Thus arcing can be detected by monitoring the current flow for the series of pulses. While arc detectors are known in the prior art, such arc detectors have limited application and are not easily adaptable to the requirements for a broadly applicable arc detector.
One example of a prior art arc detector is disclosed and claimed in U.S. Pat. No. 4,376,243 issued to Renn, et al. In the arc detector shown in Renn, which is for use with an electric rod furnace, a shunt resistor is placed in the current path and the voltage across the shunt resistor is applied to an opto-isolator. The output of the opto-isolator is then used as the input to an arc detection circuit. The arc detection circuit includes a current change detector and a preset counter to count the number of instances where the current change is above a threshold. However, because of its application, the Renn arc detector does not need to address power usage, noise, or cost concerns.
With a shunt resistor, power is continually lost in the electrical system due to the voltage drop across the shunt. The shunt described in Renn consumes up to 500 watts, an amount which is insignificant compared to the 650 kilowatts consumed by the furnace. However, in normal, everyday applications power loss is more critical and shunt is impractical. Another problem with the shunt resistor is that it is not isolated from the electrical system, which is likely a high voltage system. Consequently, the isolation is limited to the isolation provided by the opto-isolator. Opto-isolators typically have low linearity, although this is less of a problem when dealing only with the DC currents in Renn. Also, with DC currents, AC filtering need not be addressed, since any AC currents are likely to be arc pulses.
Noise and the cost of the arc detector are also not a problem in the arc detector of Renn. Given the cost and size of the typical electric rod furnace, they are likely to be permanently installed in a controlled environment, such as a factory or a foundry. In this controlled environment, it is a small matter to eliminate sources of electrical noise, since most of the personnel near the electric rod furnace are likely to be working on the electric rod furnace and not operating AC equipment near the arc detector. The arc detector of Renn assumes a DC current, and a typical AC current sources would continually trigger such an arc detector. Thus, the only noise likely to interfere with the arc detector is the arc noise signal being detected. However, if the Renn arc detector were used in a home application, where a hand-held drill, vacuum cleaner or a washing machine motor might be expected to generate electrical noise very similar to an arc signal, an arc detector coupled to a conduction path must be immune to such noise sources, while still remaining feasible at a low cost.
Cost, and therefore circuit complexity, is also not a concern in the Renn detector, since even the most complex arc detection circuit will not be as complex as the circuits for controlling the furnace itself, and the cost of any such arc detector circuit would be insignificant given the cost of the furnace.
From the above it is seen that an improved arc detection system is needed. An improved arc detection system must be low cost, and therefore must comprise few components, must be capable of operating in noisy environments, and must not drain significant power from the circuit being monitored.